Laser Ignition of Energetic Materials

The book gives an introduction to energetic materials and lasers, properties of such materials and the current methods for initiating energetic materials. The following chapters and sections highlight the properties of lasers, and safety aspects of their application. It covers the properties of in-service energetic materials, and also materials with prospects of being used as insensitive ammunitions in future weapon or missiles systems or as detonators in civilian (mining) applications. Because of the diversity of the topics some sections will naturally separate into different levels of expertise and knowledge.

About the Authors xiii

Preface xv

Acknowledgements xvii

1 Historical Background 1

1.1 Introduction 1

1.2 The Gunpowder Era 2

1.3 Cannons, Muskets and Rockets 2

1.3.1 Musketry 7

1.3.2 Rocketry 9

1.4 Explosive Warheads 9

1.5 Explosives Science 11

Bibliography 14

2 Review of Laser Initiation 17

2.1 Introduction 17

2.2 Initiation Processes 19

2.3 Initiation by Direct Laser Irradiation 21

2.3.1 Laser Power 21

2.3.2 Laser Pulse Duration 22

2.3.3 Absorbing Centres 22

2.3.4 Pressed Density 23

2.3.5 Strength of Confining Container 24

2.3.6 Material Ageing 25

2.3.7 Laser-Induced Electrical Response 25

2.4 Laser-Driven Flyer Plate Initiations 25

2.5 Summary and Research Rationale 27

2.5.1 Rationale for Research 28

Bibliography 29

References 29

3 Lasers and Their Characteristics 35

3.1 Definition of Laser 35

3.2 Concept of Light 36

3.3 Parameters Characterizing Light Sources 39

3.4 Basic Principle of Lasers 45

3.5 Basic Technology of Lasers 47

3.6 Comparison between Laser and Thermal Sources 48

3.7 Suitable Laser Sources for Ignition Applications 49

3.7.1 Nd:YAG Laser 50

3.7.2 Light Emitting Diodes (LEDs) 50

3.7.3 Diode Lasers 52

3.8 Beam Delivery Methods for Laser Ignition 53

3.8.1 Free Space Delivery 53

3.8.2 Fibre Optics Beam Delivery 54

3.9 Laser Safety 57

3.9.1 Laser Interaction with Biological Tissues 57

3.9.2 Precaution against Ocular Hazards 57

Bibliography 59

4 General Characteristics of Energetic Materials 61

4.1 Introduction 61

4.2 The Nature of Explosions 61

4.3 Physical and Chemical Characteristics of Explosives 63

4.4 Fuel and Oxidizer Concept 64

4.4.1 Explosive Mixtures 66

4.4.2 Pyrotechnics 69

4.4.3 Rocket Propellants 73

4.5 Explosive Compounds 74

4.5.1 Chemical Classification 74

4.6 Thermodynamics of Explosions 80

4.6.1 Oxygen Balance 82

Appendix 4.A 83

A.1 Data for Some Explosives 83

A.1.1 TNT (Trinitrotoluene) 83

A.1.2 HNS(Hexanitrostilbene) 83

A.1.3 DATB (1,3,Diamino,2,4,6,trinitrobenzene) 84

A.1.4 TATB (1,3,5,-Triamino-2,4,6-Trinitrobenzene) 84

A.1.5 Picric Acid (2,4,6,trinito- hydroxy benzene) 84

A.1.6 Styphnic Acid (2,4,6,trinito-1,3, dihydroxy benzene) 84

A.1.7 Tetryl or CE (Composition Exploding) 85

A.1.8 PICRITE (Niroguanidine) 85

A.1.9 RDX (Research Department eXplosive) 85

A.1.10 HMX (High Molecular-weight eXplosive) 85

A.1.11 EGDN (Nitroglycol) 86

A.1.12 NG (Nitroglycerine) 86

A.1.13 NC (Nitro-Cellulose) 86

A.1.14 PETN (Pentaerythritol Tetranitrate) 87

A.1.15 Metal Salts 87

A.2 Unusual Explosives 88

A.2.1 Tetrazene 88

Bibliography 89

5 Recent Developments in Explosives 91

5.1 Introduction 91

5.2 Improvements in Explosive Performance 91

5.2.1 Heat of Explosion ΔHc (Q) 91

5.2.2 Density of Explosives 92

5.3 Areas under Development 92

5.3.1 New Requirements for Explosive Compositions 93

5.4 Plastic-Bonded High Explosives 95

5.4.1 Plastic-Bonded Compositions 95

5.4.2 Thermoplastics 96

5.4.3 Thermosetting Materials 96

5.5 Choice of High Explosive for Plastic Bonded Compositions 97

5.6 High-Energy Plastic Matrices 97

5.7 Reduced Sensitivity Explosives 99

5.8 High Positive Enthalpies of Formation Explosives 101

5.8.1 High Nitrogen-Containing Molecules 102

5.8.2 Pure Nitrogen Compounds 102

5.8.3 Other High-Nitrogen Compounds 104

5.8.4 Nitrogen Heterocycles 105

Glossary of Chemical Names for High-Melting-Point Explosives 113

Bibliography 113

References 113

6 Explosion Processes 117

6.1 Introduction 117

6.2 Burning 117

6.3 Detonation 123

6.4 Mechanism of Deflagration to Detonation Transition 124

6.5 Shock-to-Detonation 127

6.6 The Propagation of Detonation 128

6.7 Velocity of Detonation 129

6.7.1 Effect of Density of Loading 131

6.7.2 Effect of Diameter of Charge 131

6.7.3 Degree of Confinement 131

6.7.4 Effect of Strength of Detonator 132

6.8 The Measurement of Detonation Velocity 133

6.9 Classifications of Explosives and Pyrotechnics by Functions and Sensitivity 133

6.10 The Effects of High Explosives 135

6.10.1 Energy Distribution in Explosions 135

6.11 Explosive Power 137

6.12 Calculation of Q and V from Thermochemistry of Explosives 138

6.12.1 General Considerations 138

6.12.2 Energy of Decomposition 138

6.12.3 Products of the Explosion Process 139

6.13 Kistiakowsky - Wilson Rules 140

6.14 Additional Equilibria 141

6.15 Energy Released on Detonation 142

6.16 Volume of Gases Produced during Explosion 144

6.17 Explosive Power 145

6.17.1 Improving Explosives Power 146

6.18 Shockwave Effects 147

6.19 Appendices: Measurement of Velocity of Detonation 149

Appendix 6.A: Dautriche Method 149

Appendix 6.B: The Rotating Mirror Streak Camera Method 151

Appendix 6.C: The Continuous Wire Method 152

Appendix 6.D: The Event Circuit 152

Bibliography 153

References 153

7 Decomposition Processes and Initiation of Energetic Materials 155

7.1 Effect of Heat on Explosives 155

7.2 Decomposition Mechanisms 162

7.2.1 Thermal Decomposition Mechanism of TNT 163

7.2.2 Non-Aromatic Nitro Compounds 164

7.2.3 Nitro Ester Thermal Decomposition 167

7.2.4 Nitramine Thermal Decomposition 168

7.2.5 Photon-Induced Decomposition Mechanisms 169

7.3 Practical Initiation Techniques 172

7.3.1 Methods of Initiation 173

7.3.2 Direct Heating 174

7.3.3 Mechanical Methods 175

7.3.4 Electrical Systems 177

7.3.5 Chemical Reaction 177

7.3.6 Initiation by Shockwave 178

7.4 Classification of Explosives by Ease of Initiation 178

7.5 Initiatory Explosives 179

7.5.1 Primary Explosive Compounds 179

7.5.2 Primer Usage 181

7.6 Igniters and Detonators 182

7.7 Explosive Trains 183

7.7.1 Explosive Trains in Commercial Blasting 187

Bibliography 190

References 190

8 Developments in Alternative Primary Explosives 193

8.1 Safe Handling of Novel Primers 193

8.2 Introduction 193

8.3 Totally Organic 194

8.4 Simple Salts of Organics 199

8.5 Transition Metal Complexes and Salts 202

8.6 Enhancement of Laser Sensitivity 206

References 207

Appendix 8.A: Properties of Novel Primer Explosives 211

Appendix 8.B: Molecular Structures of Some New Primer Compounds 213

Purely Organic Primers 213

9 Optical and Thermal Properties of Energetic Materials 221

9.1 Optical Properties 221

9.1.1 Introduction 221

9.1.2 Theoretical Considerations 222

9.1.3 Practical Considerations 225

9.1.4 Examples of Absorption Spectra 226

9.2 Thermal Properties 231

9.2.1 Introduction 231

9.2.2 Heat Capacity 232

9.2.3 Thermal Conductivity 232

9.2.4 Thermal Diffusivity 233

References 234

10 Theoretical Aspects of Laser Interaction with Energetic Materials 235

10.1 Introduction 235

10.2 Parameters Relevant to Laser Interaction 236

10.2.1 Laser Parameters 236

10.2.2 Material Parameters 236

10.3 Mathematical Formalism 237

10.3.1 Basic Concept 237

10.3.2 Optical Absorption 238

10.3.3 Optical Reflection 240

10.4 Heat Transfer Theory 240

References 245

11 Laser Ignition – Practical Considerations 247

11.1 Introduction 247

11.1.1 Laser Source 248

11.1.2 Beam Delivery System 249

11.2 Laser Driven Flyer Plate 249

11.3 Direct Laser Ignition 250

11.3.1 Explosives 251

11.3.2 Propellants 259

11.3.3 LI of Pyrotechnic Materials 263

References 267

12 Conclusions and Future Prospect 269

12.1 Introduction 269

12.2 Theoretical Considerations 269

12.3 Lasers 270

12.4 Optical and Thermal Properties of Energetic Materials 271

12.5 State of the Art: Laser Ignition 271

12.6 Future Prospect 272

References 274

Index 275

Dr S Rafi Ahmad founded and led the Centre for Applied Laser Spectroscopy (CALS) within the Department of Applied Science, Security and Resilience, Cranfield University from 1988 to 2013. He has been active for the last 3 decades in managing/supervising many R&D projects and PhD research students in the field of directed laser and applied laser spectroscopy. Dr Ahmad has authored 52 peer-reviewed publications in scientific journals, and co-authored a book with Dr Cartwright.

Dr Michael Cartwright works on novel explosive compounds and the design of safer formulations and disposal of time expired and unexploded ordnance. He graduated in Chemistry from London University in the 1960s. His first employment was with the UKAEA at Windscale and Calder Hall establishment examining analytical methods for novel nuclear fuels and processing technologies. He researched on sterilisation methods for the Milton Division of Vick International followed by research in nuclear damage processes in solids and organo-metallic chemistry at the University of Bath before moving to Cranfield University at the Royal Military College of Science in 1986. Dr Cartwright has authored over 80 papers in refereed journals and published conference proceedings, and a co-authored book.